The present invention relates to data transmission procedures, and particularly procedures involving multiple phase modulated digital signals.
In data transmission systems it is usually necessary to transmit, in addition to the useful information, certain code words of fixed content and agreed-upon meaning, known as unique words, for example to define those certain points in time which identify the beginning of a long sequence of signals or an address.
At the receiving end, these unique words must be recognizable with the greatest possible certainty. This is made difficult due to noise which is superposed on the signals as well as superposed interference signals, mainly when transmission takes place in multiple phase modulation. With two-phase, or quadrature, modulation the identification of unique words is more dependable.
In the system described by A. Ogawa and M. Ohkawa under the title "A New Eight-Phase Modem System for TDMA" as contribution D4 to the conference report of the Second International Conference on Digital Satellite Communication, Paris, France, November, 1972, such unique words are transmitted in two-phase modulation. A special two-phase demodulator, which has its input connected in parallel with the input of the multiple-phase demodulator, is provided to receive these unique words. While this arrangement seems to be unacceptably expensive, the fabrication expense for the transmitting end is relatively low.
With m phase modulation, where m generally is a power of 2, a number equal to the logarithm to base 2 of m, or 1d m, consecutive bit positions of the data to be transmitted are combined into a code word and during transmission each one of the m possible code words is identified by an assigned phase position of the carrier frequency with respect to a reference frequency. The phase angles of the modulated signals may be even numbered or, as is more commonly the case, odd numbered multiples of the angle .pi./m. At the receiving end, the stepping clock pulse can be recovered with greater certainty from signals whose phase angles are odd-numbered multiples of the angle .pi./m. It is known and customary to obtain the odd-numbered multiples from the even-numbered multiples of angle .pi./m by axis transformation.
Two-phase signals for the unique code word result if the latter is designed so that with the combination of 1d m bit positions only two given code words result, in the simplest case if a whole-number multiple of 1d m successive bit positions are identical in the unique word, i.e., if the stepping clock pulse for the unique word is 1d m times longer than the stepping clock pulse for the data to be transmitted.